In the design of turbines or compressors of the like, especially those of high speed, it is understood that close tolerance between the tips of the blades and the surrounding shroud or housing which seals one side of the blades from the other is desirable. Such a seal reduces the return flow of fluid from the high pressure to the low pressure side. The closer the shroud surrounds the tips of the blades, the more efficient is the turbine or compressor. Aerodynamic losses are also reduced by closer fitting of the blade tips to the shroud.
Nevertheless the clearance dimensions are dynamic. They increase and decrease with temperature and with mechanical and aerodynamic forces and may increase or decrease faster than the rotor. The clearance may also decrease, for example in one direction, under so-called shock loading when an airplane makes a sudden maneuver that subjects the parts to unusual acceleration forces, causing a differential expansion or movement of the parts.
The prior attack on such problems has been to design the shroud to fit closely, say within 20 to 30 mils (i.e. about 5 to 7 mm.) about the tips of the blades at ambient temperature. Moreover the shroud about the blade has been designed to be wearable or abradable relative to the blade tips. Then if there is a thermal transient or shock loading that causes a blade tip to strike the shroud, the blade material flakes off or abrades the shroud material, which may be a sprayed coating or sintered material of low density. Thus the shroud material is abradable (or wearable) with respect to the blade material. However such wear is not readily achieved in practice. By the arrangement of the present invention the blade tip may be designed to rotate in close proximity to the shroud without fear of cataclysmic destruction of either blade or shroud, should contact between the two occur due to thermal transient or shock loading.